1. Field of the Invention
The present invention relates to fiber optic sensors, and more particularly, relates to fiber optic interferometers for sensing, for example, rotation, movement, pressure, or other stimuli.
2. Description of the Related Art
A fiber optic Sagnac interferometer is an example of a fiber optic sensor that typically comprises a loop of optical fiber to which lightwaves are coupled for propagation around the loop in opposite directions. After traversing the loop, the counterpropagating waves are combined so that they coherently interfere to form an optical output signal. The intensity of this optical output signal varies as a function of the relative phase of the counterpropagating waves when the waves are combined.
Sagnac interferometers have proven particularly useful for rotation sensing (e.g. gyroscopes). Rotation of the loop about the loop's central axis of symmetry creates a relative phase difference between the counterpropagating waves in accordance with the well-known Sagnac effect, with the amount of phase difference proportional to the loop rotation rate. The optical output signal produced by the interference of the combined counterpropagating waves varies in power as a function of the rotation rate of the loop. Rotation sensing is accomplished by detection of this optical output signal.
Rotation sensing accuracies of Sagnac interferometers are affected by spurious waves caused by Rayleigh backscattering. Rayleigh scattering occurs in present state-of-the-art optical fibers because the small elemental particles that make up the fiber material cause scattering of small amounts of light. As a result of Rayleigh scattering, light is scattered in all directions. Light that is scattered forward and within the acceptance angle of the fiber is the forward-scattered light. Light that is scattered backward and within the acceptance angle of the fiber is the back-scattered light. In a fiber-optic gyroscope (FOG), both the clockwise and the counterclockwise waves along the sensing coil (referred to here as the primary clockwise and primary counterclockwise waves) are scattered by Rayleigh scattering. The primary clockwise wave and the primary counterclockwise wave are both scattered in respective forward and backward directions. This scattered light returns to the detector and adds noise to the primary clockwise wave and to the secondary counterclockwise wave. The scattered light is divided into two types, coherent and incoherent. Coherently scattered light originates from scattering occurring along the section of fiber of length Lc centered around the mid-point of the coil, where Lc is the coherence length of the light source. This scattered light is coherent with the primary wave from which it is derived and interferes coherently with the primary wave. As a result, a sizeable amount of phase noise is produced. Forward coherent scattering is in phase with the primary wave from which it is scattered, so it does not add phase noise. Instead, this forward coherent scattering adds shot noise. The scattered power is so small compared to the primary wave power that this shot noise is negligible. All other portions of the coil produce scattered light that is incoherent with the primary waves. The forward propagating incoherent scattered light adds only shot noise to the respective primary wave from which it originates, and this additional shot noise is also negligible. The dominant scattered noise is coherent backscattering. This coherent backscattering noise can be large. The coherent backscattering noise has been reduced historically by using a broadband source, which has a very short coherence length Lc. With a broadband source, the portion of backscattering wave originates from a very small section of fiber, namely a length Lc of typically a few tens of microns centered on the mid-point of the fiber coil, and it is thus dramatically reduced compared to what it would be with a traditional narrowband laser, which has a coherence length upward of many meters. See for example, Hervé Lefèvre, The Fiber-Optic Gyroscope, Section 4.2, Artech House, Boston, London, 1993, and references cited therein.
Rotation sensing accuracies are also affected by the AC Kerr effect, which cause phase differences between counterpropagating waves in the interferometers. The AC Kerr effect is a well-known nonlinear optical phenomena in which the refractive index of a substance changes when the substance is placed in a varying electric field. In optical fibers, the electric fields of lightwaves propagating in the optical fiber can change the refractive index of the fiber in accordance with the Kerr effect. Since the propagation constant of each of the waves traveling in the fiber is a function of refractive index, the Kerr effect manifests itself as intensity-dependent perturbations of the propagation constants. If the power circulating in the clockwise direction in the coil is not exactly the same as the power circulating in the counterclockwise direction in the coil, as occurs for example if the coupling ratio of the coupler that produces the two counterpropagating waves is not 50%, the optical Kerr effect will generally cause the waves to propagate with different velocities, resulting in a non-rotationally-induced phase difference between the waves, and thereby creating a spurious signal. See, for example, pages 101-106 of the above-cited Hervé Lefèvre, The Fiber Optic Gyroscope, and references cited therein. The spurious signal is indistinguishable from a rotationally induced signal. Fused silica optical fibers exhibit sufficiently strong Kerr nonlinearity that for the typical level of optical power traveling in a fiber optic gyroscope coil, the Kerr-induced phase difference in the fiber optic rotation sensor may be much larger than the phase difference due to the Sagnac effect at small rotation rates.
Silica in silica-based fibers also can be affected by magnetic fields. In particular, silica exhibits magneto-optic properties. As a result of the magneto-optic Faraday effect in the optical fiber, a longitudinal magnetic field of magnitude B modifies the phase of a circularly polarized wave by an amount proportional to B. The change in phase of the circularly polarized wave is also proportional to the Verdet constant V of the fiber material and the length of fiber L over which the field is applied. The sign of the phase shift depends on whether the light is left-hand or right-hand circularly polarized. The sign also depends on the relative direction of the magnetic field and the light propagation. As a result, in the case of a linearly polarized light, this effect manifests itself as a change in the orientation of the polarization by an angle θ=VBL. This effect is non-reciprocal. For example, in a Sagnac interferometer or in a ring interferometer where identical circularly polarized waves counterpropagate, the magneto-optic Faraday effect induces a phase difference equal to 2θ between the counterpropagating waves. If a magnetic field is applied to a fiber coil, however, the clockwise and counterclockwise waves will in general experience a slightly different phase shift. The result is a magnetic-field-induced relative phase shift between the clockwise and counterclockwise propagating waves at the output of the fiber optic loop where the waves interfere. This differential phase shift is proportional to the Verdet constant. This phase difference also depends on the magnitude of the magnetic field and the birefringence of the fiber in the loop. Additionally, the phase shift depends on the orientation (i.e., the direction) of the magnetic field with respect to the fiber optic loop as well as on the polarizations of the clockwise and counterclockwise propagating signals. If the magnetic field is DC, this differential phase shift results in a DC offset in the phase bias of the Sagnac interferometer. If the magnetic field varies over time, this phase bias drifts, which is generally undesirable and thus not preferred.
The earth's magnetic field poses particular difficulty for Sagnac interferometers employed in navigation. For example, as an aircraft having a fiber optic gyroscope rotates, the relative spatial orientation of the fiber optic loop changes with respect to the magnetic field of the earth. As a result, the phase bias of the output of the fiber gyroscope drifts. This magnetic field-induced drift can be substantial when the fiber optic loop is sufficiently long, e.g., about 1000 meters. To counter the influence of the magnetic field in inertial navigation fiber optic gyroscopes, the fiber optic loop may be shielded from external magnetic fields. Shielding comprising a plurality of layers of μ-metal may be utilized.